A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes. (google)
This is not all. From Oracle I know that if you want a full table scan or above 70% the index delay the retrival. If less is ok.
In general we use index to find the proper record(s) that must manage (update,delete,view), and in many cases this become faster with index.